Renal arteriovenous fistula (AVF) is an uncommon clinical entity, which can be congenital, acquired, or idiopathic. Diagnosis is aided by radiological studies, with digital subtraction angiography as a gold standard. However, Doppler ultrasound with color Doppler interrogation and computed tomography (CT) angiography are alternative imaging techniques that are noninvasive and can be used for patients to whom no intervention is planned. In this report, we present a case of idiopathic renal AVF diagnosed by Doppler ultrasound and CT angiography. Grayscale ultrasound showed a cyst-like lesion which was confirmed to be a focal aneurysmatic dilatation with heterogeneous fill-in and turbulent blood flow on color Doppler and spectral analysis. CT angiography demonstrated multiple aneurysmatic dilatations of the segmental branch and early opacification of right renal vein on the arterial phase, consistent with renal AVF.